FTR, note that efreeti do have a dimensional travel abilities. All genies can plane shift at will, to the Astral plane, the Prime Material, or any of the Elementals. (This is easy to overlook; it's under the general Genie heading.) But dimensional anchor probably stops that anyhow.
The easiest way to prevent cheap wishing is by using the line in planar binding that says, "impossible demands or unreasonable commands are never agreed to." The DM can declare that asking an Efreet to grant wishes under duress is unreasonable. Perhaps, for cultural reasons, Efreet take wishes extremely seriously, and flatly refuse to give them out except on their own volition. Binding an efreet to grant a wish would be like binding a paladin to sell his grandmother into slavery; he'll fight to the death first.
If you don't like that idea, then you can revert to the old standby of Turnabout Is Fair Play. That's a bit more complicated, but it has an air of poetic justice that I quite like.
It starts with a mental picture. Imagine that ordinary human NPCs are being magically kidnapped from the middle of a large city in your game world. Every so often a citizen suddenly vanishes without trace, is gone for a day or two, and then just as suddenly reappears in the spot he left. All these victims tell the same story about extraplanar spellcasters who imprisoned them, threatened them with death, and forced them to work as slaves for a short time.
A few people never come back at all. Magical divinations reveal that they attempted to resist their captors, and were slain.
If this started happening in a major human city, the authorities would be understandably upset about it. What is their most probable response? They hire some adventurers, of course!
Your players may get a couple of wishes out of their first couple of kidnappings, but they're in for a rude shock when a party of genie adventurers scry them and teleport into their base in the middle of the night. A couple of raging Efreeti Brb10 are quite a handful, especially when backed up by their buddies of other classes.
In order to avoid being a total bastard, you may want to give your PCs warning of this ahead of time. Tell the wizard's player that he has heard of Bad Things happening to people who abuse outsiders-- all of them end up dead under mysterious circumstances. If he insists on going ahead with the scheme anyway, he deserves the beatdown that he's gonna get.